Study on structures and interactions of lipid rafts using model cell membrane

Abstract

Lipid rafts are cholesterol-rich nano-domain within cellular membranes that are known to have more ordered lateral organization than surroundings. They provide solid supports for a number of membrane associated proteins, thus are deeply associated with diverse cellular processes from signaling transduction pathways to disease developments. However, it is highly challenging to study the physical properties of lipid rafts through actual cells due to their small size and complexity of cell membranes. Therefore, many types of model membrane that mimic the lipid raft have been proposed and extensively studied over past decades. In this dissertation, we will study the structures and interactions of lipid rafts using the model membrane. Specifically, structures and interactions of lipid rafts in lipid multilayers formed by spin coating method were analyzed by synchrotron X-ray reflectivity, fluorescence imaging and fluorescence correlation spectroscopy. In the first part of the thesis, the structures and phase transition of model lipid multilayer, composed of DPPC and cholesterol which are main components of lipid rafts, were observed by X-ray reflectivity and fluorescence imaging. It is found that the DPPC multilayer undergoes phase transition and the vertical structure changes over cholesterol concentration. As the second part, detailed lateral structure of lipid raft is investigated through the dynamics measurement. We find the unique diffusion dynamics of same DPPC and cholesterol multilayer by using fluorescence correlation spectroscopy and fluorescence recovery after photobleaching. Fast, anomalous and length dependent non-Brownian motion at low cholesterol concentration which might originated from boundary lines between subdomain structure is observed. For the study on the interactions between lipid rafts, the out-of-plane alignment and in-plane growth of raft domains in phase-separating lipid multilayer, which consists DPPC, DOPC and cholesterol, are analyzed. In particular, we revealed that there is specific interaction between the raft domains across the different bilayers using unique ability of $Ca^{2+}$ ion that can control the interlayer spacing. In addition, we suggest novel underlying mechanism how lipid rafts in distinct bilayers can interact each other through molecular dynamics simulation.